1. Field of the Invention
The invention relates generally to conductor routing among circuit elements in an integrated circuit. More particularly, the invention relates to structure and a method which facilitate changing circuit operation in any one of the conductor layers in an integrated circuit.
2. Description of the Prior Art
Integrated circuits ("ICs") typically have a substrate, an active region on the substrate containing a large plurality of circuit elements such a transistors, resistors, capacitors etc., and a region adjacent the active region which has a plurality of conductor layers and insulating layers interspersed between the conductor layers. The conductor layers each include a large number of conductor tracks which are used to provide power to the circuit elements, as well as for signal routing between the elements.
During design and testing of an integrated circuit, or even after an IC had been in production, it is often necessary or desirable to revise the circuit operation to remove faults or to otherwise improve circuit operation. At the most severe end of the spectrum of revisions, the changes may require modifications to the masks which define the content and arrangement of circuit elements on the substrate. More frequently, however, the designer has anticipated potential problems and has included sufficient circuit elements on the integrated circuit to fix the problem. For example, the designers may include a selection of buffer circuit elements to remove timing problems in the signal routing. It is then a matter of modifying the conductor routing so as to decouple and/or couple certain of the circuit elements so as to implement the fix.
Additionally, many chip designs incorporate an identification ("HID") module which is readable by software. The purpose of the module ID is to allow software to identify the hardware and, based on the ID, configure the chip and the corresponding software. As the industry moves to systems on a chip design, where proven circuit modules are "plugged into" the IC, it will be desirable for each module to have a corresponding ID module.
When making a circuit fix/enhancement to a particular module, it will typically be desirable to change the output of the module ID circuit, so that software will read a different ID code reflecting the change.
Traditional implementations of ID modules is done by Register Transfer Level ("RTL") implementation of a module ID register. In other implementations, the ID module is not a register at all, but simply constants which can be read by the software. In the case of a constant, the ID can not be changed by altering routing in one or more of the conductor layers, and a full new mask set may be required to implement the change in the ID. This is expensive in terms of cost of the mask set as well as in the schedule slippage required by the time to create new masks.
Other implementations allow the value of the module ID to be revised through changes in routing in the metal layers. However, in these known techniques, the required changes in the routing are typically in layers which are different than the layers used to implement the circuit fix which necessitated the revision of the module ID. Thus, additional mask changes are required.